See Incus / LXD profile for GUI apps: Wayland, X11 and Pulseaudio for fresh instructions.
By using proxy devices in Incus, you make available the host’s Unix sockets for Pipewire to processes in the container. You are loosening up the isolation of the container by giving out access to those Unix sockets. The Pipewire service on the host does not realize that a process in some container has become yet another first-class consumer of audio.
In the container you only need the client part of the audio software to run, because you are placing carefully the live Unix socket that directs to the host. A process in the container will use the client libraries of PipeWire to access the Pipewire Unix socket, and send/receive audio.